The present invention relates generally to a communication system and, more particularly, to an asynchronous transmission system for binary-coded information.
Generally in systems for transmitting binary-coded information, there are two types of transmission systems: synchronous transmission systems, and asynchronous transmission systems. According to the synchronous transmission system, a transmission signal is in synchronization with a clock. Usually, continuation of a signal having the same polarity is restricted by scrambling the signal or by encoding a transmission line. Because of the clock signal of the synchronous transmission system the mark density of the transmission line is almost 50 percent. Therefore, the average level (direct-current level) of the transmission signal is almost constant and a data signal of each polarity appears at least one time for a certain period of time, which facilitates stable automatic gain control (AGC) in a receiving terminal and monitoring in a transmission line. However, in the asynchronous transmission system, the 50% mark density and the restriction of continuation of the same polarity signal cannot be expected. Therefore, if the data to be transmitted comprises successive data of the same code "0" or "1", the transmission signal includes a direct-current component. As a result, the average level of the transmission signal fluctuates greatly, so that AGC in a receiving terminal is difficult and, in addition, monitoring the transmission line is difficult. In addition, if the data includes successive data of the same code such as "0", it is difficult to discriminate an interruption of the transmission line from the successive data "0".
According to one conventional asynchronous transmission system, in a transmitting terminal, asynchronous data is synchronized by using sampling pulses so that the signal is converted into a RZ (Return to Zero) coded signal or other coded signal. As a result, the direct-current component of the transmission line current is diminished. However, the conventional asynchronous system requires a complex electrical structure for the transmitting and receiving terminals.
According to another conventional asynchronous transmission system for binary-coded information, in a transmitting terminal, a pulse is generated at each transition of a signal and in addition, if there are no changes in the level of the binary-coded signal for a predetermined period of time, a refresh pulse is added to the pulse-coded signal so as to diminish the direct-current component of the transmission line current. In more detail, for example, when the binary-coded signal is changed from the "0" level to the "1" level, a positive-going pulse is generated. Similarly, when the binary-coded signal is changed from the "1" level to the "0" level, a negative-going pulse is generated. In addition, if there are no changes in the level of the binary-coded signal for a predetermined period of time, a refresh pulse, the polarity of which is the same as that of the last generated pulse, is generated. Therefore, there are three levels for the pulse-coded signal: a negative voltage, an off-condition level and a positive voltage in the case where the pulse-coded signal is electrical. In particular, in the case where the pulse-coded signal is optical, the three levels thereof correspond to an off-condition level, about a half brightness level and a full brightness level (see: U.S. Pat. No. 4,027,152).
However, the last-mentioned conventional system has the following disadvantages.
(1) The transmitting terminal is complex in electrical structure, since the transmitting terminal must generate the three levels of electrical or optical signals. Simultaneously, the receiving terminal is also complex in electrical structure, since the receiving terminal must discriminate between the three levels.
(2) In the case where the pulse-coded signal is optical, the operation of the transmitting and receiving terminals is performed for half of the full brightness, so that the margin of operation of the circuits used for the system becomes small.
(3) A refresh pulse may be added to the pulse-coded signal directly even before a change thereof, so that the transmission rate of the pulse-coded signal becomes substantially larger than the maximum transmission rate of the transmission line. As a result, data transition may be extinguished which creates a high possibility for the generation of data errors.